Critical reflooding of spent fuel packages

The transport of spent fuel in type B fissile packages is done dry, without the presence of water in the package. This measure relates to demonstrations done in the safety analysis report concerning radiolysis, criticality and pressure aspects related to water contents. This measure implies that after the loading of the packages in spent fuel pools, the packages are drained and dried.In case there is a non-conformity of the package relative to the package approval certificate or an operational problem causing the transport to be delayed or annulled, the package often needs to be unloaded or re-opened. To unload the spent fuel or re-open the package lid, the package needs to be reflooded. This is an unusual and critical procedure for most NPPs since the unloading normally occurs at the receiving end: reprocessing or waste facility.The fuel in the package has increased in temperature after the drying process. The adding of relatively cold water could cause a thermal shock for the spent fuel and the production of steam is to be avoided. This can cause a pressure rise in not only the package but also in the installation. By re-adding water in the spent fuel package, neutronic moderation is increased and precautions have to be taken to avoidcriticality problems.After reflooding the spent fuel cask, there can be a need for cooling. The temperature of the spent fuel has risen due to the dry atmosphere in the cask. Before re-immersion in the spent fuel pool and to avoid thermal shock, the spent fuel could be cooled inside the package. The Transnubel experience has shown that this operation can be executed safely with the proper precautions and preparations.